The present invention relates to an interspinous prosthesis intended to provide disc support and to dampen the relative movements of two adjacent vertebrae during the flexion or extension movements of the spinal column.
French Patent No. 94. 03716, filed in the name of the Applicant, describes a prosthesis comprising a body made of flexible material and two rigid, V-shaped inserts. The body is intended to be inserted between the spinous apophyses of two vertebrae in order to maintain a flexible anatomical spacing between them, while at the same time permitting their relative movement. The inserts make it possible to delimit grooves for receiving the spinous apophyses of the two vertebrae and they comprise a means for fixing the prosthesis to these apophyses.
The prosthesis according to said earlier patent is satisfactory in practice, but it has become evident that it could be improved, particularly as regards its structure and its resistance to the repeated stresses to which it is subjected. This is because the joins between the body of the prosthesis and the inserts are the focus of considerable stressing.
French Patent Application No. 2,623,085 describes an H-shaped wedge structure which has, on only one of its two end faces, or on both, a groove which is dimensioned to receive, with slight lateral play, a respective spinous apophysis. Each groove is delimited by lips of small height which, viewed from the side, have the shape of an arc of a circle.
The object of the present invention is to make available an interspinous prosthesis ensuring perfect support of the two vertebrae concerned, and consequently permitting perfect disc support, which prosthesis has to be able to resist the repeated stresses exerted on it by the apophyses, has to be designed so as to undergo only minimal wear, and has to exhibit considerable stability in all directions, in particular with respect to xe2x80x9clateral tiltxe2x80x9d movements, that is to say movements provoked by torsion of the spine on its axis.
The prosthesis which is the subject-matter of the present patent application is, in a manner known per se, made of a material which is multi-directionally flexible and elastic, and comprises an interspinous portion having a thickness slightly greater than the anatomical interspinous space when the spine is in lordosis, such that this portion is compressed slightly when the prosthesis is placed between the spinous apophyses of two vertebrae.
According to the invention, the prosthesis has two pairs of lugs projecting longitudinally on either side of its interspinous portion, these lugs having substantial heights in relation to the total height of the prosthesis, namely, for each pair of lugs, of the order of 30 to 45% of this total height, each pair of lugs being integral with the said interspinous portion and delimiting a deep recess which is able to receive the corresponding spinous apophysis without play, with a wide surface area of contact between these lugs and this apophysis.
These deep recesses ensure that the prosthesis is held perfectly in position between the spinous apophyses of the vertebrae, in all directions, and in particular with respect to movements resulting from torsion of the spine on its axis, which movements tend to create a lateral tilting of the prosthesis.
Moreover, and in partiuclar, the combination of this structure of multi-directionally flexible and elastic material, on the one hand, and of these two pairs of lugs of substantial length forming a unit with the interspinous portion of the prosthesis, on the other hand, means that when this interspinous portion is under compression, the lugs can be made to bear against the lateral faces of the spinous apophyses, by means of a xe2x80x9cself-closingxe2x80x9d effect.
This holding of the prosthesis, allied with the flexibility of this prosthesis, permits substantial reduction in the friction between the prosthesis and the apophyses, which renders unnecessary the inserts of rigid material in the prosthesis according to the prior art. The prosthesis according to the invention is thus monobloc, which also solves the problems associated with manufacture, and especially with the strength of the prosthesis according to the prior art over the course of time.
The prosthesis according to the invention combines an effect of suppressing the contacts newly created between the facets following intense bearing, and an effect of reducing the intradiscal pressure, permitting a slowing down in the ageing of the disc.
The main indications for this prosthesis are:
arthropathy of the facets;
prevention of disc degeneration occurring subsequent to arthrodesis;
xe2x80x9creliefxe2x80x9d of the disc ring remaining after surgical treatment of a herniated disc.
The internal faces of two lugs of one and the same pair of lugs are preferably inclined in such a way as to converge towards one another in the direction of the bottom of the recess which they delimit.
A relative wedging of the apophysis is thus obtained by slight elastic deformation of the lugs, which contributes to holding the prosthesis in relation to the apophyses.
The lugs advantageously have an average thickness which is relatively great in relation to the average width of the prosthesis, namely, for each lug, of the order of 25 to 35% of this average width.
These lugs are thus perfectly integral with the interspinous portion of the prosthesis, which ensures their resistance to the repeated stresses to which the latter is subjected.
Preferably, the anterior face of the prosthesis connects respectively with the upper and lower faces of the prosthesis via slanted and/or rounded zones, permitting the total absence of the angles which these pairs of faces would otherwise form.
The prosthesis can thus be placed in the area of the base of the spinous apophyses, at the lamina/spinous process junction of the vertebrae, which reduces the extent of the torsional stresses which the apophyses are capable of exerting upon it.
The prosthesis is advantageously bored with at least one transverse conduit formed in the area of its interspinous portion, this conduit permitting the engagement of a cord intended to join the prosthesis tightly to at least one of the spinous apophyses.
These cords are simply intended to secure the positioning of the prosthesis, the latter being self-closed between the apophyses as a result of its aforementioned structure.
Preferably, the wall of the interspinous portion which delimits this conduit is widened at the ends of this conduit, in order to eliminate any sharp edge capable of creating a point of wear of the said cord.
According to a preferred embodiment of the invention in this case, the prosthesis comprises two transverse conduits, each receiving a cord for connecting it to the spinous apophysis of the corresponding vertebra.
The prosthesis according to the invention is advantageously placed in a textile sheath which matches its shape.
This sheath avoids direct contact between the synthetic material from which it is made, in particular silicone, and the surrounding tissues, and it facilitates the incorporation of the prosthesis with these tissues. In addition, it constitutes a means of limiting the stretching of the prosthesis, eliminating any risk of the latter breaking under exceptionally high loads.
This sheath advantageously comprises a small strip sewn onto it on the posterior side of the prosthesis, which can serve as an anchoring point for a prosthetic ligament replacing the interspinous and supraspinous ligament.